THE SURFACE OF THE EARTH. 



In the like manner, there is just so much of the northern polar 

 region enlightened, and of the southern polar region darkened, in 

 July, as there is of the northern polar region darkened, and the 

 southern enlightened, in January. The diurnal and nocturnal 

 vicissitudes, therefote, of the northern frigid zone in July, are 

 identical with those of the southern frigid zone in January, and 

 vice versa. 



If the reader will take the trouble of following the position of 

 the earth from month to month, as shown in fig. 12, he will be 

 able to satisfy himself, that at all intervals of six months the 

 vicissitudes of light and darkness are in the same way reci- 

 procated between the two frigid zones. 



It might be inferred, that the continual presence of the sun 

 above the horizon, would necessarily produce a great calorific 

 effect ; and at least during that portion of the year, during 

 which they are respectively inclined towards the sun, the polar 

 circles would enjoy intense heat. That such, however, is not 

 the case is easily explained. The heat imparted by the sun 

 to any part of the earth exposed to its influence, depends, as 

 already stated, on two conditions : first, the altitude to which 

 it rises, and secondly, its continuance above the horizon, or the 

 length of the day. But the former condition has a vastly 

 greater influence upon the thermal effects than the latter. 

 Although, therefore, the continuance of the sun above the 

 horizon, at the polar circles, is favourable to the development 

 of heat, yet the very low altitude to which it rises counter- 

 acts this effect; so that within the polar circles, even with 

 the influence produced by the continuous presence of the sun, 

 the general temperature is invariably below that at which 

 water freezes. 



It is from this continuance of a temperature so low that the 

 frigid zone has taken its name. 



173. Torrid Zone. The part of the earth included between 

 the tropical circles, which comprises all places having a latitude 

 less than 23^, is called the Torrid Zone. 



Although the exposure of these regions to the heat of the sun 

 varies within certain limits between December and June, being most 

 completely presented to that luminary in March and September, 

 they, nevertheless, receive the solar influence in a much more 

 powerful degree, than the parts of the globe having higher lati- 

 tudes in the one hemisphere and the other. 



It is demonstrated in physics, that the heating power of the 



sun's rays falling upon any object increases in proportion as they 



approach to the perpendicular direction upon it. At the time of 



the equinoxes, therefore, the noon-day sun upon the Line, project- 



174 



